1. Field of the Invention
The present invention relates to a sheet feeding apparatus that feeds stacked sheets one at a time and an image forming system including the sheet feeding apparatus.
2. Description of the Related Art
Conventionally, some image forming systems have been configured by connecting a sheet feeding unit, an insertion apparatus, a post-processing apparatus, and so on to an image forming apparatus such as a printer or a copying machine. The body of the image forming apparatus, the sheet feeding unit, and the insertion apparatus in the image forming system are provided with a sheet feeding apparatus for separating one of sheets stacked on a sheet stacking unit from the other sheets and feeding the sheet to convey the sheets to an image forming unit or the post-processing apparatus. An example of the sheet feeding apparatus is an air sheet feeding type for blowing air from side surfaces of sheets stacked on a sheet stacking unit to float the sheets and adsorbing the uppermost sheet onto a suction conveyance belt to convey the sheet, as discussed in U.S. Pat. No. 5,645,274.
FIG. 9 is a schematic view of an example of the sheet feeding apparatus of the air sheet feeding type.
The sheet feeding apparatus includes a tray 12 on which sheets P are stacked, a leading edge regulating plate 17 for regulating a leading edge position in a sheet feeding direction of the sheets P, and a trailing edge regulating plate 13 for regulating a trailing edge position of the sheets P. The sheet feeding apparatus further includes a side edge regulating plate 14 for regulating a position in a direction perpendicular to the sheet feeding direction of the sheets P (a sheet width direction). The regulating plates respectively position the front, rear, and right and left of the sheets P stacked on the tray 12. The sheet feeding apparatus further includes an air blowing unit for blowing air from a side of a sheet bundle stacked on the tray 12, and a suction conveyance unit arranged in its upper part for adsorbing and conveying the uppermost sheet.
The air blowing unit includes a blowing nozzle 33 for blowing air on the top of the sheet bundle stacked on the tray 12 from the side to float several top sheets P in the sheet bundle. The air blowing unit further includes a separation nozzle 34 for blowing air between the uppermost sheet floated by the air blown from the blowing nozzle 33 and the sheets P under the uppermost sheet to separate the uppermost sheet from the other sheets P.
A suction conveyance unit includes a suction conveyance belt 21 for adsorbing the sheet P and conveying the adsorbed sheet P rightward in FIG. 9, a suction duct 51 arranged inside the suction conveyance belt 21, and a suction fan 36 for making the pressure in the suction duct 51 negative. The suction conveyance belt 21 is provided with a lot of suction holes. Air is sucked in from the suction holes by the negative pressure in the suction duct 51 so that the uppermost sheet is adsorbed on an attraction surface of the suction conveyance belt 21. The suction duct 51 is provided with an adsorption completion detection sensor 19 for detecting that the sheet P has been adsorbed on the suction conveyance belt 21.
The position in the sheet feeding direction of the sheets P stacked on the tray 12 is regulated by the leading edge regulating plate 17 and the trailing edge regulating plate 13. Air blown in a direction D in FIG. 9 by the blowing nozzle 33 floats the several top sheets P in the sheet bundle, and air blown in a direction E in FIG. 9 separates one sheet P from the other sheets by the separation nozzle 34 so that the sheet P can be adsorbed on the suction conveyance belt 21. Furthermore, the suction fan 36 brings the inside of the suction duct 51 into a negative pressure condition so that the sheet P can be adsorbed on the suction conveyance belt 21. After the adsorption completion detection sensor 19 detects the adsorption of the sheet P on the suction conveyance belt 21, the adsorbed sheet P is conveyed rightward in FIG. 9 by rotating the suction conveyance belt 21.
In recent years, a need to produce a booklet has been increased. Tab-attached sheets may be used as a partition of the booklet. In a sheet feeding apparatus of an air sheet feeding type, the need to feed the tab-attached sheets has arisen.
In the sheet feeding apparatus, the setting direction on the tray 12 of the tab-attached sheets differs depending on an image formation position of an image forming apparatus and a processing position of a post-processing apparatus connected to the downstream side of the image forming apparatus. When the tab-attached sheets with their tab portions on the leading edge side are fed, the tab-attached sheets are set on the tray 12 with the tab portions on the downstream side in the sheet feeding direction. When the tab-attached sheets with their tab portions on the trailing edge side are fed, the tab-attached sheets are set on the tray 12 with the tab portions on the upstream side in the sheet feeding direction.
When the tab-attached sheets are set with the tab portions on the upstream side in the sheet feeding direction, if the length in a width direction (a direction perpendicular to the sheet feeding direction) of the trailing edge regulating plate 13 is small, the tab portions may not be regulated depending on the position, so that the tab-attached sheets are shifted backward. As proposed in Japanese Patent Application Laid-Open Gazette No. 2000-229732, this problem is solved by attaching a tab guide member that can abut on all tab portions of set tab-attached sheets, to a trailing edge regulating plate.
When the tab-attached sheets set with the tab portions on the downstream side in the sheet feeding direction are fed, however, the following problem occurs. FIG. 10 is a lower perspective view of the suction conveyance unit in a case where the tab-attached sheets S are thus set and the uppermost tab-attached sheet SA. FIG. 11 is an upper perspective view of the suction port 52 of the suction duct 51 and the tab-attached sheet S.
The suction port 52 is formed on a lower surface of the suction duct 51, and is opposed to the suction holes of the suction conveyance belt 21. When the pressure in the suction duct 51 becomes negative, air is sucked in from the suction holes of the suction conveyance belt 21 via the suction port 52. Thus, the uppermost tab-attached sheet SA is adsorbed onto the suction conveyance belt 21.
Generally, the tab portion of the tab-attached sheet S is formed with its one part projected, at its edge. Furthermore, a position formed along the edge of the tab-attached sheet S is shifted for each sheet. When the tab-attached sheet S is set with the tab portion on the downstream side in the sheet feeding direction, as illustrated in FIG. 10, if the uppermost tab-attached sheet SA is adsorbed, the suction port 52 of the suction duct 51 cannot be block with the uppermost tab-attached sheet SA. Therefore, a part of the suction port 52 is opposed to not only the uppermost tab-attached sheet SA but also the subsequent tab-attached sheet. When air is sucked in from the suction holes of the suction conveyance belt 21 with the pressure in the suction duct 51 made negative in this state, respective tab portions of tab-attached sheets SB, SC, SD, . . . under the uppermost tab-attached sheet SA are adsorbed, as illustrated in FIG. 11. When the sheets S under the uppermost tab-attached sheet SA are thus adsorbed, two or more of the sheets S are adsorbed on the suction conveyance belt 21 and conveyed. Therefore, double feeding (a phenomenon that sheets are fed in an overlapping state) occurs.
A portion 52A indicated by oblique hatching in FIG. 11 is a portion, of the suction port 52 of the suction duct 51, which cannot be block with the uppermost tab-attached sheet SA.
When the tab-attached sheets S are set with their tab portions on the downstream side in the sheet feeding direction, an area, which overlaps with the tab portion, of the suction port 52 of the suction duct 51 is block. As a result, the respective tab portions of the tab-attached sheets SB, SC, SD . . . under the uppermost tab-attached sheet SA are prevented from being adsorbed, and double feeding is avoided. If this configuration is adopted, however, a distance L between the leading edge regulating plate 17 and the suction port 52 of the suction duct 51 is lengthened. In this portion at the distance L, the tab-attached sheets S cannot be adsorbed. Therefore, a range in which the leading edge side of the sheets cannot be adsorbed, is widened. When sheets other than the tab-attached sheets S are fed, if the sheets are thin sheets having low rigidity, for example, their leading edges hang. When the suction conveyance belt 21 starts to convey the sheets, therefore, the sheets are caught by a guide or the like and jamming occurs. In other words, the suction port 52 of the suction duct 51 must be extended close to the leading edge regulating plate 17 as much as possible so that the leading edges of the sheets adsorbed on the adsorption conveyance belt 21 do not hang. When the suction port 52 is extended, however, double feeding of the tab-attached sheets S occurs, as described above.